// This file is part of Eigen, a lightweight C++ template library
// for linear algebra.
//
// Copyright (C) 2018 Rasmus Munk Larsen <rmlarsen@google.com>
//
// This Source Code Form is subject to the terms of the Mozilla
// Public License v. 2.0. If a copy of the MPL was not distributed
// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.

// Barrier is an object that allows one or more threads to wait until
// Notify has been called a specified number of times.

#ifndef EIGEN_CXX11_THREADPOOL_BARRIER_H
#define EIGEN_CXX11_THREADPOOL_BARRIER_H

namespace Eigen {

class Barrier
{
public:
    Barrier(unsigned int count) : state_(count << 1), notified_(false) { eigen_plain_assert(((count << 1) >> 1) == count); }
    ~Barrier() { eigen_plain_assert((state_ >> 1) == 0); }

    void Notify()
    {
        unsigned int v = state_.fetch_sub(2, std::memory_order_acq_rel) - 2;
        if (v != 1)
        {
            // Clear the lowest bit (waiter flag) and check that the original state
            // value was not zero. If it was zero, it means that notify was called
            // more times than the original count.
            eigen_plain_assert(((v + 2) & ~1) != 0);
            return;  // either count has not dropped to 0, or waiter is not waiting
        }
        std::unique_lock<std::mutex> l(mu_);
        eigen_plain_assert(!notified_);
        notified_ = true;
        cv_.notify_all();
    }

    void Wait()
    {
        unsigned int v = state_.fetch_or(1, std::memory_order_acq_rel);
        if ((v >> 1) == 0)
            return;
        std::unique_lock<std::mutex> l(mu_);
        while (!notified_) { cv_.wait(l); }
    }

private:
    std::mutex mu_;
    std::condition_variable cv_;
    std::atomic<unsigned int> state_;  // low bit is waiter flag
    bool notified_;
};

// Notification is an object that allows a user to to wait for another
// thread to signal a notification that an event has occurred.
//
// Multiple threads can wait on the same Notification object,
// but only one caller must call Notify() on the object.
struct Notification : Barrier
{
    Notification() : Barrier(1){};
};

}  // namespace Eigen

#endif  // EIGEN_CXX11_THREADPOOL_BARRIER_H
